1. Field of the Invention
This invention relates generally to optical temperature compensation of spectral modulation sensors, and, more particularly, to spectrographic interrogation of spectral modulation sensors.
2. Description of the Related Art
In advanced aircraft applications, the use of fiber optic sensors to carry information between sensors and control modules provides immunity from contamination by electromagnetic sources, reduces volume and weight by eliminating the need for electromagnetic shielding, and further reduces weight by replacing metal conductors with low weight optical fibers. Advantages afforded by spectral modulation based fiber optic sensors include polarization independence, ease of multiplexing sensors with a common electro-optical interface, and reduced sensitivity to variations of link losses.
The sensitivity of these sensors to source temperature, however, severely limits their usefulness for aircraft engine or airframe applications. Typically, light emitting diodes (LEDs) are used for interrogating the sensors. The source temperature drift is critical in view of the high temperatures and acute temperature changes that occur in aircraft engines. The process of using thermo-electric coolers for controlling the source temperature is slow and breaks down at high temperatures due to the diffusion of carriers and electro-migration in the thermo-electric element.
To avoid measurement inaccuracies resulting from changes in light source intensity and changes in light transmission intensity due to bending of the optical fibers or optical connector loss, Saaski et al., U.S. Pat. No. 4,945,230, issued Jul. 31, 1990, describes a technique using ratiometric measurement with spectral modulation sensors. In the Saaski device the physical parameter being measured causes changes in the reflectivity and transmission of the sensor's optically resonant structure and thus spectrally modulates the output light from the sensor as a function of the physical parameter being measured. The spectrally modulated output light is converted into an output electrical signal by detection means. In one embodiment the detection means splits the spectrally modulated light into two spectral components, each of which is separately converted into an electrical signal by a photodetector means. A divider circuit then takes the detector ratio of these two electrical signals to provide an output signal. The Saaski device, however, does not avoid inaccuracies due to source temperature.
Aforementioned co-pending Berkcan, Ser. No. 08/118,467 (attorney's docket number RD-21,991), discloses a device employing a source monitoring component which receives light from the source, compensates the light, and then splits the compensated light into two spectral components which are converted into electrical signals by photodetectors. A source ratio including the excitation light converted into the electrical signals is taken in addition to taking a detector ratio including two components of spectrally modulated light converted into electrical signals. The source and detector ratios are multiplied to determine a detected ratio which can be used with a calibration curve to compensate for inaccuracies in measurement due to source temperature variations.
It would be desirable to have a detection system in which the source light can be sent to an optical processing component in an unmodified form and yield measurements of increased precision.